1. Field of the Invention
The present invention relates to an output feedback frequency control device for a rotating machine speed control.
2. Description of the Related Art
A general frequency control method used in a power plant is a PID control method that feeds back an angular velocity deviation, its integral, and its differential value by using a suitable gain constant. However, differential control is greatly affected by noise. Therefore, differential control is scarcely used, due to the instability of a differential signal, PI control being chiefly used instead.
However, when the instability of a differential signal is eliminated, frequency control performance can be greatly improved. Therefore, a scheme using an approximate differential signal df/dt≈Δf/Δt has been proposed, and a modified PID control method has also been proposed.
FIG. 1 is a block diagram of a frequency control system using the modified PID controller.
The structures and operations of the functional blocks are disclosed in Korean Patent Publication No. 2001-0010437.
However, in the modified PID control method, the control effect depends on the value of Δt in approximate differentiation. If the Δt value is excessively small, it is difficult to solve the problem of instability. It has been proved that the modified PID control has excellent performance. When the feedback gain (1/R) of a frequency deviation is increased, control is destabilized. However, the modified PID control can secure stability even when the feedback gain (1/R) of a frequency deviation is increased. Also, the modified PID can greatly improve the damping effect of a control system. Thus, if the problem of differential signal instability is solved, the PID control including a differential signal can secure an excellent control effect in speed governor control.
As the complexity of a power system increases, system stabilization becomes more important. A long-term oscillation is an example of system destabilization. A long-term oscillation is correlated with a frequency feedback gain (1/R), and the frequency feedback gain (1/R) is reduced to prevent the long-term oscillation. However, if the frequency feedback gain (1/R) is excessively small, it may solve a portion of the long-term oscillations but the control effect is not efficient enough. A power system stabilizer (PSS) for system stabilization by exciter control has been developed as a fundamental solution thereof and is being widely used. However, because the PSS is controlled on the basis of a linear model in the vicinity of an operation point, it requires frequent tuning of various parameters. If a significant oscillation lasts long time, it may be amplified to cause system instability. However, if the tuning is incorrect, the PSS control may cause a negative effect. In the event of a sudden system change caused by an accident, the tuning is difficult to perform. Also, when the system is in an emergency situation, a heavy load may be applied to a power generator. In this case, an exciter cannot afford to accept a PSS control signal, because the exciter may also be overloaded to control a voltage (In the 2003 North American Blackout, many power generators in Toronto were tripped due to the overloading of exciters).
Thus, PSS-based system stabilization may fail to have noticeable effects in an emergency situation. However, modified PID control can greatly improve power generator damping. Modified PID control can greatly improve system stabilization in association with the PSS. In particular, the modified PID control can secure greater effects when the PSS malfunctions.